The present invention generally relates to a disc variation detecting apparatus, and in particular to a disc variation detecting apparatus for detecting variations in an inclination or a position in an axial direction of an information recording disc upon recording or reproduction.
An optical disc is well known as an information recording disc. The optical disc is widely used as an information recording means of a high information recording density. However, the optical disc has difficulty in permanently maintaining its flat plane. In general, the relative height relationship between center and outer periphery parts of the disc varies, and the disc is thus distorted in the form of a dish. In particular, an optical disc with an air sandwiched type configuration is liable to have the inclination occurring at disc planes in inner and outer periphery parts having a spacer.
When recording information on a distorted disc, a spot of a light beam is deformed on an information recording surface of the disc, because the light beam cannot be perpendicularly projected thereon due to distortion of the disc. Therefore, there is the possibility that distorted pits are formed on the information recording surface and some pits are lacking. Deformation of the pits causes degradation of the frequency characteristic upon reproduction. The occurrence of inclination at the inner periphery part of the disc is a particularly serious problem where the disc is rotated so that an angular velocity of the disc is constant. On the other hand, upon reproduction, a beam of light which is emitted by a head or pickup and is reflected on the disc surface, does not return to the pickup accurately, so the reproduction of the recorded information is degraded.
In order to resolve the above problems, a disc inclination detecting apparatus has been proposed in the Japanese Pat. Laid-Open Publication No. 236,130/1985, for example. This apparatus has a pair of a light-emitting element and a light-sensitive element which are located in the radial direction of the disc. A light beam emitted by the light-emitting element is reflected on the surface of the disc. A reflected light beam reaches the light-sensitive element, and moves on a light-sensitive surface of the light-sensitive element in a direction parallel to the axis of the disc, corresponding to the inclination of the disc. Then, the pickup is attitude-controlled by an attitude-controlling servo system by the positional displacement of the reflected light beam on the light-sensitive surface.
Another disc inclination detecting apparatus is also known. This conventional apparatus includes a light-emitting element and two light-sensitive elements. These elements are arranged in the radial direction of the disc. The light-emitting element emits each of two light beams towards the inner and outer periphery of the disc. The light beams are each reflected by the disc and received by the corresponding light-sensitive elements. Then, the voltage difference between the outputs of the light-sensitive elements is amplified by a differential amplifier. The amplified difference voltage varies with the inclination of the disc.
However, the former prior disc inclination detecting apparatus aforementioned has the following disadvantage. The reflected light beam moves on the light-sensitive surface in the direction parallel to the axis of the disc, even when the disc itself moves in the axial direction and correspondingly the distance between the surface of the disc and a pair of the light-emitting element and the light-sensitive element is varied. Therefore, the apparatus detects the movement of the disc in the axial direction as if the disc is inclined. It should be noted that this movement of the disc does not substantially affect the recording or reproducing operation.
The latter conventional disc inclination detecting apparatus has the following disadvantages, when applied to a write-once type optical disc. With this type of disc, the reflectivity of the light beam at a recorded part on which an information signal has been recorded is different from the reflectivity at an unrecorded part on which an information signal has not yet been recorded. When the recorded part of the recording surface is positioned inside a position on the recording surface which corresponds to the position of the light-emitting element, and when the unrecorded part is positioned outside that position on the recording surface, the difference voltage indicating the inclination of the disc is outputted even if the disc has no inclination, that is, the disc is kept horizontal.
In order to overcome the problems especially in the former prior disc inclination detecting apparatus, a novel and useful disc inclination detecting apparatus was just previously proposed in a U.S. patent application Ser. No. 094,113 filed on Sept. 8, 1987, U.S. Pat. No. 4,805,164, and entitled "DISC INCLINATION DETECTING APPARATUS", the assignee of which is the same as the present application. This proposed disc inclination detecting apparatus may not be totally eliminate the problems in the latter conventional apparatus.